Polyamide materials are widely used commercially. Polyamide linkages are found in a wide variety of fibers and fabrics, such as wool, silk, natural leather, synthetic leather, and nylon. Wool is composed primarily of alpha-keratin, a naturally occurring alpha-helical fibrous protein. Silk is composed primarily of beta-keratin, a naturally occurring fibrous protein existing in a zig-zag structure. Leather is almost pure collagen, a fibrous protein composed primarily of glycine, alanine, proline and 4-hydroxyproline, forming a 3-strand helical structure. Nylon is a synthetic polyamide prepared by the polycondensation of a dicarboxylic acid and diamine, such as adipic acid and hexamethylene diamine (nylon 6,6). Nylon is also produced from a cyclic amide such as caprolactam (nylon 6). Nylon fiber is widely used to made textiles, and in particular, is used to make a substantial amount of the world's carpet.
Although polyamide materials have favorable attributes which make polyamide materials desirable for many applications, one disadvantage is that polyamide materials are easily stained. Fluorochemical coatings have been developed for imparting stain resistance to polyamide fiber carpets. Fluorochemicals used to impart stain resistance typically contain a perfluoroalkyl radical having 3-20 carbons, and are produced by condensation of a fluorinated alcohol or fluorinated primary amine with a suitable anhydride or isocyanate. For example, N-ethyl perfluorooctyl-sulfonamidoethanol and toluene diisocyanate can be reacted in a 2:1 molar ratio to produce a stain resistant fluorochemical. Fluorochemical coatings resist wetting of the carpet surface and minimize chemical contact between the carpet surface and staining materials and also provide a physical barrier to the staining material. Although fluorochemical coatings are effective in protecting polyamide materials from being stained by certain materials such as soil, fluorochemical coatings are not particularly effective to prevent staining by acid dyes which are found in substances such as mustard, wine, coffee, and soft drinks. FD&C red dye number 40 is an acid dye found in many food and drink products and is a common cause of carpet staining. Acid dyes stain polyamide materials such as nylon fibers by forming ionic bonds with protonated terminal amine groups on the polyamide material surface.
A conventional treatment for increasing the resistance of polyamide materials from staining by acid dyes has been to coat the polyamide materials with a sulfonated hydroxyaromatic formaldehyde condensation polymer. Such polymers include sulfonated phenol formaldehyde condensate polymer and sulfonated dihydroxy diphenyl sulfone formaldehyde condensation polymer. These materials provide stain resistance by bonding to the surface of the polyamide materials. The sulfonate groups of the sulfonated hydroxyaromatic formaldehyde condensation polymers bond to available protonated amino groups on the polyamide material surface thereby preventing the protonated amino groups from bonding to acid dyes.
Although sulfonated hydroxyaromatic formaldehyde condensation polymers are effective stain resistant treatments, one drawback is that such coatings discolor over time by gradually turning yellow when exposed to ultraviolet light or nitrogen dioxide. This is a particularly serious problem when such materials are used on light-shaded textile articles.
Therefore, there is a need for a stain resistant treatment for polyamide materials that is effective to prevent staining by acid dyes and does not yellow overtime.